In order to control the flight of aerodynes such as helicopters, it is necessary for the blades of the rotor to be acted upon so as to vary their pitch, both collectively for all the blades, in order to control the lift of the rotor, and cyclically, at each rotation, in order to adjust the inclination of the rotor's plane of rotation, and control the trim and traverse in a horizontal direction.
For this purpose, there is usually provided an oscillating plate positioned concentrically with the rotor shaft and linked to the rotor blades, the inclination of the said plate being controlled by the pilot by means of three servoactuators which determine the spatial trim.
These servoactuators are therefore essential for flight, and thus each one usually comprises, redundantly, two actuators linked together, powered by two separate hydraulic circuits, so as to guarantee manoeuvring with at least one of them, even in the event the other, or its feeding circuit, being is faulty or damaged.
For this purpose, various embodiments are possible in which the rods of the said actuators aligned with each other, are inter-connected, or else the said actuators are positioned in side-by-side relationship with the eyelet for connection to the said oscillating plate being carried by a yoke or the like, connecting the actuator rods.
The first solution, with aligned actuators, occupies little room transversely and the stresses exerted are completely symmetrical, but occupies a great deal of space longitudinally, i.e. almost twice the space necessary for a single actuator, and is thus unsuitable for some applications.
By contrast, the solution whereby the actuators are positioned side-by-side takes up more room transversely and moreover, in the event of emergency operation, with just one actuator working, there is misalignment between the direction of action of the actuator and the point of application of the resistance. This produces a bending moment in the yoke and in the actuator rod, with high stress which, under such conditions, limits the time for which they can be used safely and dictates that the parts in question be over-sized, thus considerably increasing their weight.